Database Error

Greetings All,
Am less than a month old in learning CI, I have a problem:
I'm using the form_validation library to validate form inputs.
Two of the fields (username and email fields) has to check in the DB that the user input is unique and doesn't already exist in the DB - I'm creating a user system using ion_auth. The code for the validation of the fields are as follows

PHP Code:

$this->form_validation->set_rules('name', 'Display Name', 'is_unique|isme_users.username|required');
        $this->form_validation->set_rules('email', 'Email', 'valid_email|is_unique|isme_users.email|required'); 


I'm getting the error in the attached picture.

   

I'm running LAMPP on Ubuntu and my server parameters are as follows (from PHPMyAdmin)

• Apache/2.4.29 (Unix) OpenSSL/1.0.2n PHP/7.2.0 mod_perl/2.0.8-dev Perl/v5.16.3
  
  
• Database client version: libmysql - mysqlnd 5.0.12-dev - 20150407 - $Id: b396954eeb2d1d9ed7902b8bae237b287f21ad9e $
  
  
• PHP extension: mysqli curl mbstring
  
  
• PHP version: 7.2.0
  

My knowledge of databases and SQL re li,ited and am still laerning them.
Help is needed and appreciated.